1. Field of the Invention
The present invention relates to a color reproduction definition creating method of creating a color reproduction definition representing a correspondence between a device value and an output color in a device outputting an image on a medium with a color material of each color of an amount according to the device value of each of multiple colors, and a color conversion method of converting the device values among multiple devices.
2. Description of the Related Art
Various types of input devices for inputting an image to obtain image data are known, such as a color scanner for reading a recorded image and obtaining the image data and a DSC (Digital Still Camera) for focusing the image of a subject on a solid-state image sensing device and reading it to obtain the image data. As for these input devices, the image data is represented by device values in a fixed range such as a range including 0 to 255′ for each of three colors of R (Red), G (Green) and B (Blue) for instance. As for the three colors of R, G and B, the colors representable by the device values in the fixed range are limited respectively. For this reason, once converted to the image data by using the input device, the image represented by the image data is limited to the colors in a color area equivalent to the range in a color space of R, G and B even if the colors of an original image has very rich representation.
As for output devices for outputting the image based on the image data, various types are known, such as a photo printer for exposing a photographic paper with a laser beam and developing it to record the image thereon, a printer for recording the image on paper by an electrophotographic method or an inkjet method, a printing machine for creating a large amount of printed matter by rotating a rotary press, and a light-emitting display device such as a CRT display or a plasma display for emitting light on a display screen based on the image data and displaying the image. These output devices also have the respective color representation areas as with the input devices. To be more specific, the output devices can represent various colors based on the image data representing the three colors of R, G and B and the image data representing four colors of C (Cyan), M (Magenta), Y (Yellow) and K (Black). However, the representable colors are limited to those within the color areas in which the output device color spaces (such as an RGB space and a CMYK space) exist (within the color areas represented by the device values in the range of 0 to 255 as to each of R, G and B for instance).
Such color areas representable by the input devices and output devices are called color reproduction areas (color gamut). The correspondence between the device values (coordinate values of device color spaces) and the colors in color reproduction areas which is represented by an LUT (Look UP Table) and so on is called a color reproduction definition (color profile).
In recent years, the cases of mutually converting the image data among various input devices and output devices are increasing, and there is a known isochromatic method of using the color reproduction definition to keep the colors of the image isochromatic on converting the image data. As described above, however, the color reproduction areas (color gamut) are different according to the devices. Therefore, there are the cases where, if the image data is converted by the method of merely keeping the colors of the image isochromatic, there arises lack of color representation in a portion where the color reproduction areas are mismatched so that the image represented by the converted image data becomes an unnatural image.
It is also empirically known that the images which are originally the same in spite of different color reproduction areas are representable as natural images to the human eye by each of various devices. The natural images represented by such various devices are represented mutually in a little different colors according to differences in their color reproduction areas. As the human eye is highly adaptive, the images give a natural impression whichever device they may be represented by.
Thus, there is a need for the color conversion for converting the colors of the image to keep the natural impression of the image on converting the image data. It is desirable that such color conversion is the one for associating the colors in the color reproduction area (color gamut) of a certain device with those in the color reproduction area (color gamut) of another device in just proportion. This color conversion is called gamut mapping.
Here, the case of converting the image data from one input device to multiple models of output devices is assumed for instance.
In this case, assuming that the color conversion (gamut mapping) from the color reproduction area (color gamut) of the input device as a conversion source to the color reproduction area (color gamut) of each output device as a conversion destination is individually performed, there arises a problem that the impressions of the images outputted by the multiple output devices are mutually different. Thus, there is a proposed color conversion method of creating a virtual common color reproduction area including the respective color reproduction areas of the multiple models of output devices and undergoing the color conversion from the color reproduction area of the input device to the common color reproduction area and the color conversion from the common color reproduction area to the color reproduction area of each output device (refer to Japanese Patent Laid-Open No. 2002-252785 for instance). It is expected that, by adopting such a color conversion method, the impressions of the images after the color conversion match up among the multiple devices as the conversion destinations that each have been color-converted from the device as a common color conversion source respectively.
In the case where the image data is converted as described above, the method of the color representation of the input device is often different from that of the output device, such as the color representation in the three colors of R, G and B and that in the three colors of C, M and Y for instance. And forms of the color reproduction area are significantly different between the devices of different color representation methods. If the color conversion of a general method is performed between the color reproduction areas in significantly different forms, there often arises discontinuity such as missing and crushing of tones which becomes a cause of image quality degradation. Thus, there is a proposed color conversion method whereby a change direction of the color in the color conversion (gamut mapping) is decided in the device color space so as to maintain continuity of the tones on the color conversion (refer to Japanese Patent Laid-Open No. 2001-103329 for instance).
It is thinkable, at first glance, that a combination of the color conversion described in Japanese Patent Laid-Open No. 2002-252785 and the color conversion described in Japanese Patent Laid-Open No. 2001-103329 allows the color conversion capable of maintaining the continuity of tones and causing the impressions of the images among the devices as the conversion destinations to match up.
To combine the color conversion described in Japanese Patent Laid-Open No. 2002-252785 with the color conversion described in Japanese Patent Laid-Open No. 2001-103329, however, there must exist a color reproduction definition representing a correspondence between colors in the common color reproduction area and device values. As the common color reproduction area is virtually created to include color reproduction areas of the respective devices of multiple models, there is no definition as to the correspondence to device color spaces.
As for the common color reproduction area itself, it cannot be infinitely large just because it includes the color reproduction areas of the respective multiple devices. Unless the common color reproduction area has the adequate largeness and form, it eventually causes discontinuity of tones and so on.